At present, many generally-used industrial apparatuses or consumer products include rotation units or rotation parts such as motors or gear wheels. According to progression of science and technology and an increase in social demands for protection of earth environment and energy saving, industrial apparatuses such as aircrafts or ships or consumer apparatuses such as cars try to be controlled at high accuracy and high precision. In order to control rotation machines and rotation apparatuses at high accuracy and high precision, the rotation speed or the number of rotations needs to be continuously measured accurately. Therefore, first, a simplified, small-sized, light-weighted measurement apparatus capable of measuring a rotation speed more accurately needs to be provided at low cost with mass production in order to respond to the social demands.
As methods of measuring the rotation speed or the number of rotations, a method using electromagnetic induction and a method using an optical type magnetic field sensor using the Faraday effect of a magneto-optical material (magnetic garnet) have been proposed.
As a method using electromagnetic induction, a rotation speed meter for measuring the rotation speed or the number of rotations of engines of aircrafts or cars has been commercialized. However, the rotation speed meter using the electromagnetic induction has serious problems in that electromagnetic noise may be easily received in a transmission line (cable) between a measurement terminal and a main body of the apparatus. In addition, since the rotation speed meter using the electromagnetic induction uses an electrical circuit, there is a serious problem in that hazardous material treatment facilities such as hazardous material production sites or hazardous material treating sites treating combustible materials such as organic solvents need to provide explosion proof methods.
On the contrary, in measurement of rotation speed using light, for example, in an optical type magnetic field sensor using the Faraday effect of a magneto-optical material (magnetic garnet) described above is almost not influenced by electromagnetic noise. In addition, in a site treating combustible materials such as organic solvents, explosion proof methods are unnecessary. The magnetic field sensor using the magnetic garnet uses the effect that the Faraday rotation angle of the magnetic garnet is changed under the influence of an external magnetic field. In other words, in the magnetic field sensor, the polarization plane of the light beam transmitting through the magnetic garnet is allowed to be changed according to a change in a magnetic field applied to the magnetic garnet, and the change in the polarization plane is allowed to be converted into a change in an intensity of the light beam to be sensed or counted, so that the rotation speed or the number of rotations are to be measured.
As the magnetic field sensor, there are transmission type and reflection type magnetic field sensors. In the transmission type magnetic field sensor, components need to be arranged so that the directions of incidence and transmission of a signal light beam are aligned in one straight line. Therefore, there is a limitation in an installation site to enlarge the whole magnetic field sensor in the propagation direction of the signal light beam. Accordingly, according to some use purposes or installation sites, the transmission type magnetic field sensor may not be installed and used.
As a configuration of solving the problems of the transmission type magnetic field sensor, a reflection type magnetic field sensor is proposed (for example, refer to Non-Patent Literature 1). In the reflection type magnetic field sensor 100 disclosed in Non-Patent Literature 1 illustrated in FIG. 11, a polarizer 102 is arranged in the vicinity of a magnetic garnet 101, and no optical fiber exists on an optical path between two lenses 103a and 103b. A bismuth substituted garnet having a large rotation angle with respect to a light beam having a wavelength of 1550 nm is used as the magnetic garnet 101. A thickness of the magnetic garnet 101 is set to 150 μm which is a maximum growing thickness as a single magnetic domain. In order to measure a magnetic field parallel to a surface of a measurement object, a light beam is allowed to be incident on the magnetic garnet 101 in the horizontal direction of FIG. 11, and a strength of a horizontal magnetic field is measured.
A continuous light beam having 1550 nm output from a light source (not illustrated) is used as the light beam used for magnetic field measurement. The light beam is adjusted to be a linearly polarized light beam by a polarization controller 104 and is incident on the magnetic field sensor 100. A strength of the magnetic field output from the magnetic field sensor 100 is applied to the light beam, and the light beam is converted into a voltage signal by a photo diode (PD) which is an optical receiver.